New programmable cardiac pacemakers introduces many new pacing and functions. The interactions of the heart and such pacemakers may cause dangerous arrhythmias. Possible interactions will be determined by digital computer simulation of the heart and pacemaker using a compartment model of the heart. If one includes pacemaker failure mechanism (such as runaway rates, over or under sensing, and intermittent capture) possible interactions with these new pacemakers will be numerous and difficult to predict. Even fixed-rate (VOO) pacers can produce 20 possible rhythm responses. Parameters for electrocardiologic and pacemaker functions (including wires, electrodes, connectors, and pulse generator), both normal and abnormal, will be entered in the computer. A ladder diagram will be produced indicating the response of the heart to the pacemaker. This research will enable clinicians and developers to determine if possible dangerous interactions exist.